1. Field of the Invention
This invention relates to a hydraulic circuit which improves maneuverability particularly of operation of working components of a hydraulic power shovel.
2. Related Art
As shown in FIG. 4, a hydraulic power shovel generally includes, as working members, a boom 2 with its base end pivotally supported on a main body 1, an arm 3 with its base end pivotally supported on the fore end of the boom 2, and a working tool like a bucket 4 pivotally supported at the fore end of the arm 3, turning them respectively by a boom cylinder 5, an arm cylinder 6 and a bucket cylinder 7 for performing various jobs. However, depending upon the position and posture, the weights of these working members impose rotational moments on the respective cylinders 5 to 6, forcibly extending or contracting the latter and causing the phenomenon of so-called "cavitation", i.e., creation of a vacuum state due to an outflow of oil preceding an inflow to an oil chamber on the rod- or head-side of each cylinder. The cylinders in such condition become insensible to the above-mentioned dead weights, and, even if the oil pressure is succeedingly supplied to the oil chamber, remain inoperative until the cavity or void space created by the cavitation is filled with the supplied oil. As soon as the cavity is filled, the cylinder commences operation in an abrupt manner.
These situations are explained more particularly in connection with an operation of turning the arm 3 in the direction of arrow C in FIG. 5. With regard to the cylinder 6 for the arm, it receives the rotational moment imposed by the weights of the arm 3, bucket 4, cylinder 4 and other associated parts, imposing a stretching force until the overall center of gravity G indicated by an imaginary line comes onto a vertical line y--y passing through the pivotal point of the arm 3. Accordingly, as soon as a hydraulic change-over valve 35 is shifted to position B to supply the discharge oil pressure of a hydraulic pump 8 to a head-side oil chamber 6a of a cylinder 6 through a conduit 20, the pressurized oil in the rod-side oil chamber 6b is suddenly returned to a tank 21 through conduit 19 and oil passage through the hydraulic change-over valve 35 in position B. At this time, the supply of the pressurized oil to the head-side oil chamber 6a becomes small, creating a vacuum cavity in the oil chamber. Consequently, even if extension of the cylinder 6 were continued to bring the overall center of gravity G beyond the vertical line y--y, the arm 3 would not operate until the cavity in the head-side oil chamber 6a is filled with the supplied oil pressure, the arm 3 being put in action abruptly as soon the cavity is filled.
As one can infer from FIG. 4, this phenomenon takes place not only on the vertical line y--y but also when extending the cylinders 6 and 7 from a contracted stated until the bladed end of the bucket 4 touches an object to be worked and, while continuing their extension, contracting the cylinder 5 further after contracting same until the bladed end of the bucket 4 touches the working object.
With a view to suppressing such phenomenon, the prior art proposes to provide, in the conduit 19 of FIG. 5, a slow return valve 34 which consists of a check valve and a fixed throttle valve with a throttle effect commensurate with the dead weights, the throttle valve imparting a resistance to the flow of oil which is returned from the rod-side oil chamber 6b when the arm cylinder 6 is extended, for slowing down its operating speed. Alternatively, combination relief valves 11 and 12, consisting of an overload relief valve and a check valve, are provided in conduits branched off the conduits 19 and 20, communicating the conduits 19 and 20 with a tank 21 through the check valve to prevent the cavitation.
The throttle valve which constitutes the slow return valve 34 of the prior art has no effect of preventing cavitation when its throttle effect is too low. On the other hand, when its throttle effect is too high, the operating speed of the cylinder becomes slower and unnecessary load is generated. Therefore, where adaptability to ordinary operations is concerned, it has been the general practice to make arrangements such that the cavitation would not occur to the cylinder in any marked degree at a discharge oil pressure of 60%-70% of the rated rotational speed of the engine which drives the hydraulic pump. However, recently hydraulic power shovels have been used not only for earthmoving operations in general but also for work requiring more meticulous skills like underground burying works, and it is sometimes required to replace the boom, arm and bucket to change their sizes or to set special working equipment in place of the bucket. Consequently, there have been operations where the engine speed is maintained at a low level or the load pressure on the cylinder is increased, which is difficult to cope with by the slow return valve 34 alone. Besides, as the cylinders 6 to 8 are located in positions remote from the tank 21, and the oil is taken up by the cavities in the cylinders through the lengthy conduits and check valve, the functions of the conventional combination valves 11 and 12 are often found insufficient.